Unraveling the Stereoisomer Configurations of 1,1'-bis(tert-butylphosphino)Ferrocene in the Gas Phase.

The molecular structure of a ferrocene derivative with adjacent centers of chirality, 1,1'-bis(tert-butylphosphino)ferrocene, has been examined in the gas phase using broadband microwave spectroscopy under the isolated and cold conditions of a supersonic jet. The diastereomers of 1,1'-bis(tert-butylphosphino)ferrocene can adopt homo- and hetero-chiral configurations, owing to the P-chiral substituents on the cyclopentadienyl rings. Moreover, the internal ring rotation of each diastereomer gives rise to four conformers with eclipsed ring arrangements, where the two tert-butylphosphino groups were separated by dihedral angles of approximately 72°, 144°, 216°, and 288° with respect to the two ring centers. The interconversion barriers between the conformations are below 2 kJ/mol, whereas the pyramidal inversion of the tert-butylphosphino groups is hindered by more than 140 kJ/mol, calculated at the B3LYP-D3(BJ)/def2-QZVP level of theory. In the experimental microwave spectrum, we unambiguously identified the two global-minimum diastereomers with 72° conformations. The absence of other conformers can be attributed to the relaxation dynamics in the supersonic jet, which transfers the high-energy conformers to the respective global-minimum geometries. Additionally, we discovered that London dispersion interactions between the two tert-butylphosphino groups play a crucial role in stabilizing the structures of this ferrocene complex.